1. Field of the Invention
The present invention relates to an oscillator controller and an atomic oscillator using the same. More particularly, the present invention relates to an oscillator controller for use in an atomic oscillator, as well as to an atomic oscillator whose resonance frequency derives from atomic transitions.
2. Description of the Related Art
Rubidium atomic oscillators produce a constant frequency output by utilizing atomic transitions of rubidium, the resonance frequency of which is highly stable. Because of their extremely high frequency stability, rubidium oscillators are widely used as a high-accuracy timing source for communications networks and also as a frequency standard for television broadcast services.
In rubidium oscillators, rubidium atoms in a resonance cell is pumped with lights that emanate from a rubidium discharge lamp integrated therein. FIG. 12 shows the structure of a conventional excitation circuit for energizing a discharge lamp. This circuit is composed of discrete devices connected as follows. The collector of a transistor Tr is wired to a voltage Vin, together with one end of a resistor R1. Its base is connected with the other end of the resistor R1, the anode of a diode D1, and one end of a capacitor C1, and one end of a capacitor C3. Its emitter is connected to the other end of the capacitor C1, one end of a capacitor C2, and one end of a resistor R2. The other end of the diode D1 is grounded, and so are the other ends of the capacitor C2 and resistor R2. A coil L is connected in series with the capacitor C3, with its remaining end grounded.
The coil L acts as one of the resonant elements of the above-described excitation circuit. It is wound around a piece of glassware, called a cell, in which rubidium gasses are encapsulated. This device is referred to as a rubidium lamp Lp, and the excitation circuit supplies a high-frequency current (e.g., several tens to one hundred MHz) to the surrounding coil L, causing a discharge in the rubidium lamp Lp. This produces a light, which will be referred to hereafter as xe2x80x9crubidium lamp light.xe2x80x9d The excitation circuit should control its output adaptively in accordance with the conditions of the rubidium lamp Lp. In addition, the input voltage (Vin) of the excitation circuit has to be kept as low as possible, in order to reduce the power consumption of the circuit and increase the longevity.
We have to consider some critical voltages regarding Vin, which are: start-up voltage of the excitation circuit, and turn-on and turn-off voltages of the rubidium lamp. FIGS. 13 to 15 show the relationship between those voltages and the values of capacitors used in the excitation circuit. Their horizontal axis represents the capacitor values C1 to C3. The vertical axis of FIG. 13 represents the start-up voltage, at which the excitation circuit starts to oscillate (i.e., starts to energize the rubidium lamp Lp). The vertical axis of FIG. 14 represents the turn-on voltage, at which the oscillation of the excitation circuit is strong enough for the rubidium lamp Lp to light up. The vertical axis of FIG. 15 represents the turn-off voltage, at which the rubidium lamp Lp goes out.
The curve W1 of FIG. 13 indicates a positive correlation between the capacitor value C1 and start-up voltage. This means that a smaller capacitance should be chosen for the capacitor C1 when we wish to reduce the start-up voltage. The curve W1a of FIG. 14, on the other hand, shows a negative correlation between the capacitor value C1 and turn-on voltage. This suggests to us that a larger capacitance should be chosen for the capacitor C1 when we wish to reduce the turn-on voltage.
As seen from the above explanation, there are tradeoffs between some desirable circuit characteristics, and accordingly, we have to make a compromise when deciding circuit parameters including the capacitor values C1 to C3. Think of, for example, prioritizing the reduction of start-up voltage, and select a smaller capacitance for C1 according to that policy. This choice of C1, however, results in a higher turn-on voltage and a raised turn-off voltage, which are both undesirable because it would then be more likely that the ridium lamp Lp would never turn on or might go out during the operation. Such troubles could happen after years of service, and it would be fatal if it did happen.
We may adopt another policy, giving up the benefit of low turn-on voltages. This means, however, that we have to provide a separate high-voltage pulse generator to trigger the rubidium lamp Lp, which needs several thousands to several tens of thousands of volts to start up. This pulse generator requires bulky components such as a trigger transformer, converter transformer, and charging capacitor with a large capacitance. It is therefore difficult to implement such an additional circuit in a small rubidium oscillator available in recent years.
In view of the foregoing, it is an object of the present invention to provide an oscillator controller which optimizes key circuit parameters of an excitation circuit according to the operating condition of a discharge lamp.
It is another object of the present invention to provide a highly accurate and reliable atomic oscillator which optimizes key circuit parameters of an excitation circuit according to the operating condition of a discharge lamp.
To accomplish the first object, the present invention provides an oscillator controller for use in an atomic oscillator. This oscillator controller comprises the following elements: (a) an excitation circuit having a discharge lamp, which produces a light beam by energizing the discharge lamp, the light beam being for use in pumping atoms; and (b) a circuit parameter optimizer which controls at least one circuit parameter of the excitation circuit for optimal operation thereof, comprising: a start-up voltage monitor which detects whether a start-up voltage of the excitation circuit is reached, thus producing a voltage monitoring signal; a light amount monitor which receives a resonance detection signal to check the amount of light before and after the rubidium lamp lights up, thus producing a light amount monitoring signal; and a bias voltage selector which selects a bias voltage for use in the excitation circuit, based on the voltage monitoring signal and light amount monitoring signal.
To accomplish the second object, the present invention provides an atomic oscillator whose resonance frequency derives from atomic transitions. This atomic oscillator comprises the following elements: (a) a voltage-controlled oscillator which produces an oscillation signal according to a given control voltage; (b) a frequency synthesizer which produces microwaves from the oscillation signal by modulating the oscillation signal with a low-frequency signal and upconverting the oscillation signal with frequency synthesis techniques; (c) an atomic resonator, comprising: (c1) an excitation circuit having a discharge lamp, which produces a light beam by energizing the discharge lamp, the light beam being for use in pumping atoms, (c2) a circuit parameter optimizer which controls at least one circuit parameter of the excitation circuit for optimal operation thereof, comprising: a start-up voltage monitor which detects whether the excitation circuit has reached a start-up voltage thereof, thus producing a voltage monitoring signal; a light amount monitor which receives a resonance detection signal to check the amount of light before and after the rubidium lamp lights up, thus producing a light amount monitoring signal; and a bias voltage selector which selects a bias voltage for use in the excitation circuit, based on the voltage monitoring signal and light amount monitoring signal, and (c3) a resonance detection unit which produces a resonance detection signal by detecting the amount of the light beam having passed through the atoms, the amount varying in accordance with the difference between the frequency of the microwaves and the resonance frequency of the atoms; and (d) a servo circuit which produces and supplies the low-frequency signal to the frequency synthesizer, as well as produces the control voltage for the voltage-controlled oscillator by demodulating the resonance detection signal synchronously with the low-frequency signal.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.